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<b><i>Background:</i></b> Patterns in tropical forest nitrogen cycling are poorly understood. In particular, the extent to which leguminous trees in these forests fix nitrogen is unclear.</p>
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<b><i>Aims:</i></b> We aimed to determine factors that explain variation in foliar δ<sup>15</sup>N (δ<sup>15</sup>N<sub>F</sub>) for Amazon forest trees, and to evaluate the extent to which putatively N<sub>2</sub>-fixing Fabaceae acquire nitrogen from the atmosphere.</p>
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<b><i>Methods:</i></b> Upper-canopy δ<sup>15</sup>N<sub>F</sub> values were determined for 1255 trees sampled across 65 Amazon forest plots. Along with plot inventory data, differences in δ<sup>15</sup>N<sub>F</sub> between nodule-forming Fabaceae and other trees were used to estimate the extent of N<sub>2</sub> fixation.</p>
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<b><i>Results:</i></b> δ<sup>15</sup>N<sub>F</sub> ranged from −12.1‰ to +9.3‰. Most of this variation was attributable to site-specific conditions, with extractable soil phosphorus and dry-season precipitation having strong influences, suggesting a restricted availability of nitrogen on both young and old soils and/or at low precipitation. Fabaceae constituted fewer than 10% of the sampled trees, and only 36% were expressed fixers. We estimated an average Amazon forest symbiotic fixation rate of 3 kg N ha<sup>−1</sup> year<sup>−1</sup>.</p>
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<b><i>Conclusion:</i></b> Plant δ<sup>15</sup>N indicate that low levels of nitrogen availability are only likely to influence Amazon forest function on immature or old weathered soils and/or where dry-season precipitation is low. Most Fabaceae species that are capable of nodulating do not fix nitrogen in Amazonia.</p>
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